1. Field of the Invention
This invention relates generally to electrical test equipment, and in particular, to a method and tester apparatus for verifying the load rating of an active alternating current distribution circuit.
2. Description of the Prior Art
Adequate wiring for an alternating current power distribution circuit is a function of several design considerations which involve the current capacities of conductors. Wire size is the primary factor in establishing current capacity, but the type of conductor insulation material, the proximity of other conductors, enclosure of the conductors in conduits, raceways, or thermal insulation, and ambient temperature are factors that also determine the current capacity of conductors. The National Electrical Code specifies the allowable current capacities of conductors for the various standard wire sizes as influenced by the foregoing factors.
When a power distribution circuit is designed, a wire size is selected which will carry the maximum rated current for the expected service, with a voltage drop not to exceed the amount specified by the National Electrical Code. The National Electrical Code sets guidelines to establish the maximum voltage drop allowed for a loaded circuit. It permits a two percent drop in a main feeder and a three percent drop in any branch. This maximum allowable drop is five percent for any feeder/branch combination. These guidelines are provided to prevent overheating of the wiring and to avoid damage to appliances when operated at a marginal voltage level.
Wire size, insulation condition and interconnection resistance are the main factors which contribute to excessive voltage drops in power distribution circuits. According to conventional testing procedures, a new distribution circuit is normally checked with a volt meter. Because a volt meter does not load the distribution circuit, faulty material or poor connections are not revealed. The volt meter tests only confirm that the proper connections have been made.
Even assuming that a power distribution circuit has been installed correctly, it can be damaged by excessive load conditions. An example of this is the effect of lightning striking an ungrounded T.V. antenna. Even with the T.V. turned off, the induced electric current from the lightning will pass through the T.V. set via the common conductor to the fuse box or main feeder where the common is grounded and not protected by a fuse. The on/off switch on the T.V. set disconnects only the conductor which is connected to the fuse.
Because the induced current does not pass through the fuse or circuit breaker at the fuse box, there will be no indication that an overload has occured. It is possible that the induced current may be so high that it can blister or otherwise damage the insulation around the wire. Even if the insulation is not damaged, the induced current may be high enough to alter the electrical conductivity of the wire. This type of damage appears as a sharp increase in the electrical resistance in the neutral conductor. The increase in resistance is often concentrated in small areas, forming hot spots.
Because the resistance of power conductors increases with temperature, there is a possibility that thermal runaway may occur, and set the surrounding structure on fire. Thermal runaway is an effect associated with conductors having a positive coefficient of resistance change with temperature, with a rise in temperature causing an increase in resistance which in turn causes an additional rise in temperature. When this occurs in electrical wiring, the thermal runaway will continue until the line protector trips, or the wire becomes red hot, thereby causing a fire.
Aging of the conductor also contributes to the risk of thermal runaway. As an electrical conductor ages, it takes on the electrical characteristics of a conductor which has had an excessive load applied to it, and therefore is subject to thermal runaway. The aging effect occurs at different rates for copper and aluminum power conductors. The rate that aluminum ages to exhibit a higher resistance is substantially greater than the rate at which copper ages.
The danger of thermal runaway is increased by the retrofitting of existing dwellings with thermal insulation. Any aged or overfused circuits that are covered by new insulation become confined in a thermal blanket. Under the confined conditions, the restricted heat builds up more quickly and thermal runaway happens much earlier.
Overfusing or installation of oversized circuit breakers can also bring about circuit overload which may lead to thermal runaway. An oversized fuse or breaker is often installed when the correct fuse is not available or when the original fuse repeatedly trips. Overrating may also occur from a defective breaker which will not trip. This situation is particularly dangerous since it is generally assumed (incorrectly) that an electrical circuit can be safely loaded until the circuit protector trips.
Interconnection resistance caused by defective connections also contributes to electrical wiring fires. Poor connections are usually found at wire nuts, barrier strip junctions, receptacle connections and fuse box connections. Other sources of poor connections are bad internal contacts of a circuit breaker or switch. The problem of interconnection resistance is aggravated by the use of aluminum wiring. Aluminum wiring is subject to accelerated damage from overloads, poor connections and physical damage because of the electrolysis of junctions induced by dissimilar metal reaction. Moreover, thermal expansion and contraction cause the connections to become loose. As the connections become loose, the contact resistance increases due to the reduced pressure.
It will be recognized that all wiring deteriorates and wears out in time due to the effects of current flow, switching and corrosion. Although aluminum wiring is much more likely to cause an electrical fire, copper wiring also ages and deteriorates. The useful life of any distribution circuit depends on the insulation method, its frequency of usage and the current levels to which it is subjected.
Because fuse overrating and aged wiring are common in a number of residential neighborhoods and office buildings, the only sure way to determine if an electrical fire hazard is developing in the hidden wall areas of the buildings is to test the distribution circuits at the power outlets under actual or programmed load conditions.
The terminal voltage in most private dwellings and office buildings is 120 volts RMS. The maximum allowable voltage drop from the utility power transformer to the wall outlet is five percent of RMS terminal voltage, or six volts. A deflection of six volts in a voltmeter having a full scale deflection of 150 volts represents a change of approximately four percent of full scale. Such a change in that scale range is too small to be reliably detected and accurately measured by visual inspection on most voltmeters.
There is, therefore, a serious and urgent need for a tester which can verify the load rating of an active alternating current distribution circuit in which voltage drop at the power outlet can be measured accurately and reliably by loading the distribution circuit with an actual resistance load of an appliance, or by a programmed load.